MFC r273929:

r273929 (by jmmv):

MFV: Import atf-0.21.

Import atf-0.20.

This is a MFC of r261897 and r262000 and should allow all of the atf
tests to pass.

Update atf to 0.18 and remove the code of the deprecated tools.

This is a MFC into stable/10 of:
- r258286 Update notes for imports of atf.
- r258289 MFV: Import atf-0.18.
- r258290 Drop all ATF tools code.

This is "make tinderbox" clean as run on ref10-amd64 with the default
WITHOUT_TESTS option. A "make buildworld" with WITH_TESTS set now works
as well.

MFC r273929:

r273929 (by jmmv):

MFV: Import atf-0.21.

Import atf-0.20.

This is a MFC of r261897 and r262000 and should allow all of the atf
tests to pass.

Update atf to 0.18 and remove the code of the deprecated tools.

This is a MFC into stable/10 of:
- r258286 Update notes for imports of atf.
- r258289 MFV: Import atf-0.18.
- r258290 Drop all ATF tools code.

This is "make tinderbox" clean as run on ref10-amd64 with the default
WITHOUT_TESTS option. A "make buildworld" with WITH_TESTS set now works
as well.